New generation projection lithography for integration circuits (IC) manufacturing with size of structures below 22 nm is based on usage of EUV radiation near 13.5 nm (13.5+/−0.135 nm), corresponding to reflection band of multilayer Mo/Si-mirrors. Zero defect control of IC is one of the most important metrological processes of modern nanolithography. In the case of mask defect occurrence, defects are projected onto the silicon wafer/substrate which has photoresist hence contributing to the occurrence of micro print defects on the wafer. The common approach in lithography manufacturing is moving away from IC analysis, which is excessively labor and cost intensive at high volume manufacturing, to the analysis of lithographic masks. The EUV lithography mask is a Mo/Si mirror, on top of which a topologic drawing made of material which absorbs radiation at 13.5 nm wavelength is applied. The process of mask inspection is made more effective by means of actinic radiation scanning, i.e. by radiation which has a wavelength equal to the operating wavelength of the lithography process (so called Actinic Inspection). In this case, scanning by EUV with 13.5 nm wavelength makes it possible to detect defects with resolution better than 22 nm.
Zero defect control of lithographic masks during manufacturing and operating is one of the key problems in EUV lithography. Furthermore, production of a light source at 13.5 nm for mask inspection, which has, high brightness and overall high stability, is an area of focus for EUV lithography development. Production of a relatively compact and efficient apparatus is required for the purpose. This apparatus should be on the basis of EUV source with high brightness of radiation equal to B13,5≧30 W/mm2 sr in wavelength band equal to 13.5+/−0.135 nm and small value of etendue G=5·Ω≦10−2 mm2·sr, where S-square of source in mm2, Ω-solid angle of EUV output to collector mirror in steradian.
In accordance with one of approaches, known from U.S. Pat. No. 7,307,375, issued on 12 Nov. 2007, high brightness EUV-light source is based on the inductively driven, electrodeless Z-pinch discharge. The EUV source is characterized by simplicity, compactness and relatively low cost. However, Z-pinch is produced in the SiC ceramic bush with orifice diameter 3 mm, which makes it necessary repeated periodic replacement because of significant erosion. Size of radiating plasma is relatively large, and maximal achieved brightness B13,5 amounts to ˜10 W (mm sr), which is lower than required for a variety of applications, which includes lithographic mask inspection.
The apparatus and method for EUV light generation from laser-produced plasma known from patent application US20150076359, issued on 19 Mar. 2015, don't have this flaw. In the embodiment of invention, the target material is xenon, frozen on the surface of a rotating cylinder, cooled down by liquid nitrogen. EUV radiation of laser-produced plasma is directed to an intermediate focus by the collector mirror, placed in vacuum chamber. The apparatus and method make it possible to achieve a small size of EUV emitting plasma region and high brightness of EUV source without optics contamination.
There are several disadvantages in using this technology: low efficiency of target material, high price of xenon, complicated system of recycling, necessity of high linear speed >15 m/s of cooled cylinder rotating and the related problem of EUV source stability, and lastly, the necessity of collector mirror protection from the effect of heavy ions in Xe-plasma.
The most powerful and high-efficient high-brightness LPP EUV light known, for example, from U.S. Pat. No. 7,897,947, issued on 3 Jan. 2011, contains a nozzle, a laser target, an interaction zone (where the laser hits the target) in a vacuum chamber with gas inlet, an input window for the laser beam, focused at the area of laser action, and an output of the divergent EUV-light beam to the collector mirror, placed in the vacuum chamber. In the method for EUV generation, the form of target droplets is optimized by means of laser pre-pulse and then irradiated by the main laser beam.
Similar systems with usage of tin droplet targets, irradiated by powerful pulse-repetitive CO2 laser made it possible to create most powerful EUV light sources for high-volume manufacturing of IC. However, such EUV sources are rather difficult and expensive due to the use of a complicated laser system with pre-pulse. This complication arises from the laser system along with the need for target droplet synchronization and high speed gas for cooling down and protection of the collector mirror together with its electromagnetic protection system. Periodic replacement of laser targets is used in this apparatus and method instead of a closed cycle of target material. It makes difficult the production of commercially viable EUV sources for inspection and metrology.
Applicability of Lithium (Li) as a target material is declared in U.S. Pat. No. 7,897,947 as one of the most effective target materials besides tin. White using Lithium as a laser target material, the optics are protected from contamination by means of evaporative cleaning. Heating should provide sufficient speed of Li evaporation, i.e. pressure of Li saturated steam at working temperature of optical element should exceed pressure of incoming steam.
In U.S. Pat. No. 7,525,111, issued on 3 Jan. 2011, a window of vacuum chamber with temperature of 350-450° C. is used in the apparatus of EUV generation from laser-produced plasma. However, heating of the window, necessary for its evaporative cleaning, reduces lifetime of sealing gasket and detracts from the apparatus' reliability altogether.
From publication of T. Feig et al. High-Temperature LPP Collector Mirror. Proc. of SPIE Vol. 6151, 61514A, (2006) the collector mirror, working at a temperature up to 500° C. is known. However, a thermal-stress resistant mirror is very expensive. Also, as the mirror is made of pure Mo and Si, these materials begin to react chemically with rising temperatures. The higher the temperature, the faster the reaction, which in turn promotes degeneration of the multi-layer mirror. As a result, evaporative cleaning is a factor which limits the resource of the multi-layer Mo/Si mirror.